Traditional food barrier flexible packaging materials mainly include two categories of products: those based on polyvinylidene chloride and those based on polyamide. The flexible packaging materials based on polyvinylidene chloride are more widely used, and account for most of the market. Polyvinylidene chloride has good molecular symmetry, and is liable to form crystal. Besides, polyvinylidene chloride contains a large number of polar chlorine atoms, the presence of these polar chlorine atoms greatly enhance the attraction between the molecules; thus, the molecules are tightly packed such that polyvinylidene chloride based material has a high barrier property. However, because of the large content of chlorine atoms in polyvinylidene chloride, harmful gases are produced during the disposal of polyvinylidene chloride by combustion. In recent years, with increasing environmental pressure, the development of new high barrier environmentally friendly materials in lieu of polyvinylidene chloride has become the trend.
Polyvinyl alcohol products in dry state have excellent oxygen barrier property far superior to that of polyvinylidene dichloride. However, the polyvinyl alcohol molecules contain a large number of strongly hydrophilic hydroxyl groups, which are liable to form hydrogen bonds with water in wet state, and associate with water via the hydrogen bonds. Upon the association with water, the barrier properties of the polyvinyl alcohol products will be greatly reduced. For example, polyvinyl alcohol products completely lose their original oxygen barrier property at a relative humidity of 50%, and thus can not substantially be used as a food packaging material.
In order to improve the moisture-proof property of polyvinyl alcohol, the prior art focuses on chemical modification approaches, wherein the interaction with hydrogen bonds is reduced as much as possible provided that the high barrier properties possessed by the molecular structure are maintained, and the prior art conceives solving the problem by improving the water-proof property, such that the modified polyvinyl alcohol products maintain good gas barrier properties under high humidity.
The modified polyvinyl alcohol products are quite poor in printability despite the moisture-proof is greatly improved. This is because, modified polyvinyl alcohol coating has very low adhesion to ink, as a result, the ink and coating are prone to peel off during printing. The modified polyvinyl alcohols are especially unsuitable for use in an environmentally friendly ink such as a water-based ink. On the other hand, most packaging products have to be subjected to printing; therefore, the promotion and application of the modified polyvinyl alcohol products are greatly restricted.